1. Technical Field
The present invention generally relates to devices and methods for quantifying a surface's cleanliness. In particular, the invention relates to such and methods that direct interrogating radiation toward a surface or a surface cleaner detecting radiation from the surface or the surface cleaner produced in response to the interrogating radiation.
2. Related Art
In numerous industries, surface contamination and measurement thereof are issues of utmost importance. For example, the manufacturing of painted metal articles may require metal parts to be cut before they are painted. Often, cutting processes leave oil residues on the metal parts. Oil residues may interfere with the adhesion of paint to the metal parts. Thus, as a quality control measure, the metal parts are typically cleaned after they are cut before they are painted to ensure that the paint does not peel away from the parts.
Surface purity is also an important issue in facilities that handle reactive chemicals. For example, after each run, cleanliness of pharmaceutical reactors must be validated before they are returned to use. Similarly, in environments that are repeatedly exposed to large quantities of fluorine, e.g., reaction chambers of chemical lasers that use hydrogen fluoride and/or deuterium fluoride, cleanliness is a critical issue since the fluorine compounds may be highly reactive in nature and may explode upon contact with contaminants.
Furthermore, exacting standards of surface cleanliness are required in various aerospace and astronautic applications. For example, space shuttles use large quantities of pure oxygen which explodes upon contact with hydrocarbon impurities. All surfaces exposed to pure oxygen must be cleaned to exacting standards. Similarly, many parts on modern airplanes and helicopters, particularly nonmetal articles made with epoxy composites, must be cleaned before they are bonded to each other. Otherwise, catastrophic results may occur due to adhesion failure.
In response to these concerns, various industries have come up with cleaning protocols and standards to assure address cleanliness issues. Both aerospace and semiconductor industries have found it essential to control contamination through the use of clean rooms. Clean room standards have now been adopted by the International Organization for Standardization (ISO) that set forth contamination per unit volume. As defined by ISO 14644-2, “class 100” means that there are less than 100 particles of 0.5 μm or larger in a cubic foot.
In addition, standards have been developed that set forth contamination per unit surface area. For surface contamination, standard MIL-STD-1246C (or equivalently IEST-STD-1246C) has been developed to define a surface contaminated with less than 1 microgram per cm2 of oil to be “Level A.” If a particle count is small enough to meet level 100, then is said to be level 100-A.
Nevertheless, there are a number of shortcomings associated with known protocols and standard methods for determining the cleanliness of a surface. For example, in nonvolatile residue (NVR) testing, a surface cleaner, e.g., a sheet of filter paper or (preferably non-shredding) tissue wipe, may be used to wipe down a surface. The cleaner may then be sent away to a NVR testing facility where it is washed with a suitable solvent to extract any contaminants the may be present on the tissue. Then, the solvent is left to evaporate in a preweighed weighing dish. The resulting added mass is reported in milligrams per square foot of surface area.
NVR testing is suboptimal for numerous reasons. In general, NVR testing is procedurally difficult. Such testing may also be time consuming. When the NVR testing facility is remotely located relative to the location of the surface to be tested, it may take days to receive results of such testing. To ensure that the surface does not become contaminated by the time NVR test results arrive, the surface may have to be isolated and stored in controlled environments such as clean rooms, thereby increasing the costs associated with cleanliness validation. Furthermore, such testing results occasionally in gross errors.
While in situ testing techniques are available, they are generally qualitative rather than quantitative in nature. For example, black-light (wavelength 366 nm) monitoring of large-scale bonding surface for contamination has been described in Chawla, “Measuring Surface Cleanliness,” Precision Cleaning, pages 11-15, June, 1997 (accessed from http://www.p2pays.org/ref/02/01816.htm on Mar. 2, 2009, hereinafter “Chawla”). However, such techniques are accompanies with numerous limitations. In general, black light has not been known to be useful for detecting contaminants such as light machine and tapping oils, hydraulic oil and silicone room-temperature vulcanizer (RTV) compounds that do not fluoresce strongly at low levels of contamination. In addition, black-light inspection is subjective, not quantitative and creates no record that is analyzable relative to accepted electronic standards.
In addition, fluorescent and phosphorescent methods to determine the cleanliness of metallic surfaces may not be useful for nonmetallic surfaces. For example, metallic surfaces generally do not fluorescence, whereas many composite materials used in aerospace and astronautic applications do. In some instances, the intensity of fluorescent background radiation may overwhelm the intensity of signal generated for contaminant detection.
The few quantitative technologies that have been used to determine the cleanliness of a surface other than for biohazards such as food and drink contamination are generally limited in nature. For example, U.S. Pat. No. 6,310,348 to Melling et al. describes an accessory for an FTIR spectrometer comprises fiber-optic cables that may be used to detect and characterize quantifying thin films on reflective surfaces for cleaning validation applications. Such spectrometry based technologies require a reflective surface and the precision relative placement the accessories relative to the surface so as to achieve a “grazing angle” for maximum sensitivity.
Thus, opportunities exist to provide alternatives and improvements to known protocols and technologies for evaluating the cleanliness of a surface. There exist further opportunities to provide improved technologies for evaluating the cleanliness of a surface despite the surface being comprised of a material that may produce potentially interfering background radiation, resulting in the generation of interfering noise, e.g., of a fluorescent and/or phosphorescent nature.